Heteromorphic sex chromosomes determine males and females in many species and their study is of general interest. Understanding the biases in the generation of genes and functions with respect to sex chromosomes is of primary significance to explaining sexual dimorphism and sex chromosome evolution. The sex chromosomes are found to differentiate from a pair of autosomes;most of the Y chromosome degenerates, and the X chromosome develops dosage compensation. Two main types of biases in the generation of duplicates in relation to sex chromosomes have been observed (in human, mouse and D. melanogaster): new genes are recruited by the Y chromosome and duplicates escape the X chromosome. In particular, retroposition has revealed the directionality of the duplication and has helped us to reveal duplicates that escape the X chromosome. Many of these new duplicate genes have male specific function. So the bias in their location and expression pattern reveals that significant positive selection takes place when those genes become fixed in the population and when they acquire a gender specific promoter. This project proposes two objectives: [1] to study the function and role of selection in retrogenes newly relocated from X to autosome in D. melanogaster, and [2] to study how the acquisition of male specific promoter regions takes place in these newly relocated retrogenes in Drosophila. Data of polymorphism and divergence at the nucleotide level, mRNA expression information, P element transformation technology, homologous recombination, direct mutagenesis and the most up to date bioinformatics tools and molecular evolution software will be exploited to achieve these two objectives.